Line number there is relative to what you can see at http://github.com/greg2ndQuadrant/postgres/blob/merge/src/backend/executor/execMain.c and that's a null pointer check at the beginning of EvalPlanQualFetchRowMarks. Haven't explored why this happened or how repeatable this is, but since Boxuan was looking for some bugs to chase I wanted to deliver him one to chew on.
I am not sure how this bug comes. I will try to find out the reason for it.